The optical amplifier, which is one of the most important devices for the optical communication system, can regenerate a light signal, which is attenuated while transmitting and dividing. Therefore, the development of an optical amplifier, which has a smaller volume, less noise and lower price will have a great effect on the metropolitan and the access network in the future.
In chief, three kinds of optical amplifiers are widespread in the fiber communication system, i.e. a semiconductor type, a doped fiber type and a nonlinear type. Problems of the inter-signal modulation and the distortion are easily resulted from the channel crosstalk, the four-wave mixing and the sensitivity to the polarization while the semiconductor amplifier is used for the dense wavelength division multiplexing (DWDM) system. Furthermore, a large refractive index difference between the semiconductor material and the transmission fiber leads to significant Fresnel reflection losses.
The Raman amplifier utilizing a nonlinear effect has been extensively commercialized in the recent years. The Raman amplifier has a very broad bandwidth for light amplification. However, an amplifying medium of a long-distance fiber and a high-power pumping laser are required due to the poor nonlinear coefficients of the silica fibers. As a result, the cost is expensive and the efficiency is less than those of the doped fiber amplifier. Hence the Raman amplifier dominates the wavelengths in which the doped fiber amplifier cannot operate.
Nowadays, the doped fiber amplifier is the most important and popular active device used for the DWDM system. The doped fiber, which has a dopant of rare-earth ions in the fiber, provides an optical gain of a broad wavelength range by utilizing an energy difference generating from a transition of the excited electron in the 4f electronic shell of the lanthanons series element and thereby an intensity of the light signal in the range is amplified through these excited electrons.
The erbium-doped fiber amplifier (EDFA) is the most improved one among the various doped fiber amplifiers. The working principle of the so-called EDFA is described as follows: Firstly, a pumping laser provides an appropriate initial optical source to excite the electron in the 4f electronic shell of the erbium ion from a ground state to an excited state. Then, when a light signal passes through, the transition to the ground state of the excited electron will be induced and then a photon having a same C band as the light signal will be emitted. The purpose of amplification is thus achieved.
The signal intensity will decay a lot when the DWDM signal transmits for a long distance or when the DWDM signal is divided. The intensity decaying will make the signal have a poor signal-to-noise ratio (SNR) and seriously influence the transmission quality. In order to relay the light signal in the network, the intensity of the light signal must be amplified by the EDFA after light transmitting or dividing.
Furthermore, the energy of the light signal can be directly amplified by the EDFA and a complicated optic-electro transformation is needless to involve. Such amplification has the advantages of the high amplification factor and the optical gain and the light polarization can be maintained. A simultaneous amplification for different wavelengths is hence achieved. The great development of the EDFA has improved the optical transparency of the physical layer in the optical communication and made the fiber communication break the limitation of the conventional optic-electro-optic transformation so as to enter a new age of the photonic network.
Besides, a fiber laser can be manufactured by applying the operation mechanism of the EDFA. An optical resonance system for exciting the photon is formed by installing a set of fiber gratings at both ends of the erbium doped fiber. The excited energy receives exciting emissions when the pumping light is repeatedly reflected in the fiber and the energy is amplified to generate a laser.
However, efforts to overcome the difficulties relative to the structure of EDFA are still necessary to be done. Since the modified chemical vapor deposition (MCVD) processing of the EDFA is employed, the erbium ion concentration is not easily to be controlled to prevent the up-conversion or the pair-induced quenching resulted from the uneven distribution of erbium ions, which cause a lower quantum efficiency. Therefore, the length of the erbium doped fiber must be long enough (>1 m) to achieve the amplification and thus the volume of the optical amplifier cannot be effectively reduced.
Although the pumping light and the signal light can most compactly and effectively overlap by completely doping the optical gain medium into the guiding core, the amplified spontaneous emission is limited in the guiding core and continuously amplified. This effect leads to a competition for the optical gain between the amplified spontaneous emission and the signal light and a poor SNR of the signal.
Since the application of the optical amplifier is so broad, a great effort has been done for manufacturing the optical amplifier and the laser in the field of fiber device. Several manufacturing methods are revealed and described as follows:
In 1972, a method related to an excitation by the evanescent-field of the optical waveguide for manufacturing the continuously output dye laser was disclosed by E. P. Ippen with the Bell Lab in the United States. Because there was no optical resonating system involved in this method and the refractive index of the dye was not easy to closely control to the effective refractive index of the waveguide, they had just shown that the optical amplification can be achieved based on the pumping through evanescent-field coupling.
The method to fabricate a dye fiber laser, which is related to the evanescent-pumping of a side-polished fiber, was further disclosed by H. J. Shaw et al. with the Stanford University in 1980. In this application, an optical gain of 22 dB would be obtained while using a side-polished fiber merely having an effective interaction length of 1 mm and a pulse laser. However, when the signal light beam and the pumping light beam co-propagate in the fiber, the signal evanescent-field will penetrate more because of the larger wavelength of the signal light beam. For obtaining better excitation efficiency, the pumping energy has to penetrate the optical gain medium to reach a penetrated position, which is the same as that of the signal light beam at least. Moreover, in order to get a broader excited area of the optical gain medium, the pumping light beam have to penetrate the optical gain medium as much as possible. On the other hand, in order to prevent the signal energy loss resulted from an over-penetration of the signal light, the leaky depth of the signal evanescent-field has to be limited and thus the leaky depth of the pumping light beam is not enough to excite the optical gain medium. Hence a filter is needed to filter the unused pumping power away in front of the receiver and this would result in an additional energy consuming. Therefore, it is difficult to simultaneously obtain a better exciting efficiency and an appropriate penetration of the signal evanescent-field.
Besides, the optical gain amplification was not effectively enhanced due to a poor technique for the fiber polishing and therefore the effective interaction length of the side-polished fiber was not long enough. Moreover, a temperature variation, which occurs while the dye absorbs the pumping energy, also has some influences on the refractive index of the dye. The variation of the refractive index results in a changing leaky depth of the evanescent field of the signal light beam and the excited light beam, and the optical gain obtained from the signal light is changed accordingly. Therefore the stability of the optical power output is seriously influenced.
Another kind of optical amplifier is disclosed by V. A. Kozlov et al. with the Russian Academy of Sciences (1994). The LiF crystal having a silicon dioxide coating, which has a holographic grating thereon, is worked as an optical waveguide, wherein the signal light is coupled to the optical waveguide by the holographic grating. The LiF crystal was transversely pumped by the pumping light beam in a direction from the back of the LiF crystal and hence the signal light beam would obtain the optical gain from the LiF crystal through evanescent-field coupling. However, a high-density overlap among the pumping energy, the signal energy and the optical gain medium cannot be obtained by this kind of excitation method. Thus an optical gain with high quantum efficiency is difficult to achieve. Moreover, the method of the signal incidence, which is perpendicular to the optical waveguide is not practical in the optical communication. Hence the utility of the application still needs to be considered.
A polished substrate which is made of a silicon wafer, is applied to fabricate a side-polished fiber dye optical amplifier having a long interaction length by Tseng et al. with the Tsing-Hua Univerty in Taiwan. In this application, a fiber having an effective interaction length of 5.7 mm and a relative optical gain with 95 times were demonstrated. However, the pumping efficiency is still not significantly improved because the optimal evanescent-field leaky depths of the signal and the pumping light cannot be obtained simultaneously, either.
In order to overcome the foresaid drawbacks in the prior art, an evanescent-field optical amplifier and a laser are provided in the present invention.